Outburst Photometry of the Eclipsing Dwarf Nova GY Cancri

We observed the ROSAT-selected eclipsing dwarf nova GY Cnc (=RX J0909.8+1849) during the 2001 November outburst. We refined the orbital period to be 0.17544251(5) d. The fading portion of the outburst was indistinguishable from those of typical dwarf novae with similar orbital periods. However, the signature of orbital humps (or a hot spot) was far less prominently observed in the orbital light curves and eclipse profiles than in usual dwarf novae with similar orbital periods. The combination of low frequency of outbursts and the apparent lack of slowly rising, long outbursts in GY Cnc is difficult to reconcile within the standard framework of dwarf novae. We suspect that GY Cnc may be the first above-the-gap counterpart of unusual eclipsing dwarf novae HT Cas and IR Com.Comment: 6 pages, 7 figures, to appear in Publ. Astron. Soc. Japa

Photometric Study of KR Aurigae during the High State in 2001

We photometrically observed the VY Scl-type cataclysmic variable KR Aurigae after its final rise from the fading episode in 2000-2001. Time-resolved observation revealed that the light curve is dominated by persistent short-term variation with time-scales of minutes to tens of minutes. On some nights, quasi-periodic variations with periods of 10--15 min were observed. No coherent variation was detected. The power spectral density of the variation has a power law component (f^(-1.63)). The temporal properties of the short-term variations in KR Aur present additional support for the possibility that flickering in CVs may be better understood as a result of self-organized critical state as in black-hole candidates. The light curve lacks "superhump"-type signals, which are relatively frequently seen in VY Scl-type systems and which are suggested to arise from tidal instability of the accretion disk induced by changing mass-transfer rates. The present observation suggests a borderline of superhump excitation in VY Scl-type stars between mass ratios q=0.43 (MV Lyr) and q=0.60 (KR Aur).Comment: 8 pages, 6 figures, to appear in Publ. Astron. Soc. Japa

Superhumps of CC Cancri Revisited

We observed the 2001 November superoutburst of CC Cnc. This observation makes the first detailed coverage of a superoutburst of this object. The best-determined mean superhump period is 0.075518 +/- 0.000018 d, which is 2.7% longer than the reported orbital period. This fractional superhump excess is a quite typical value for a normal SU UMa-type dwarf nova, excluding the previously raised possibility that CC Cnc may have an anomalously large fractional superhump excess. During the superoutburst plateau, the object showed a decrease of the superhump period at dot(P)/P = -10.2(1.3) x 10^(-5), which is one of the largest negative period derivative known in all SU UMa-type dwarf novae.Comment: 5 pages, 5 figures, to appear in Publ. Astron. Soc. Japa

Kilo-Second Quasi-Periodic Oscillations in the Cataclysmic Variable DW Cancri

Our photometric monitoring revealed that DW Cnc, which was originally classified as a dwarf nova (V=15--17.5), remained at a bright state of Rc=14.68+/-0.07 for 61 days. In conjunction with optical spectra lacking a strong He II emission line, we propose that the object is not a dwarf nova, but a non-magnetic nova-like variable. Throughout our monitoring, the object showed strong quasi-periodic oscillations (QPOs) with amplitudes reaching about 0.3 mag. Our period analysis yielded a power spectrum with two peaks of QPOs, whose center periods are 37.5+/-0.1 and 73.4+/-0.4 min and, furthermore, with a significant power in frequencies lower than the QPOs. DW Cnc is a unique cataclysmic variable in which kilo-second QPOs were continuously detected for 61 days. We propose two possible interpretations of DW Cnc: (i) A permanent superhumper below the period minimum of hydrogen-rich cataclysmic variables. (ii) A nova-like variable having an orbital period over 3 hours. In this case, the QPOs may be caused by trapped disk oscillations.Comment: 6 pages, 4 figures, PASJ in pres

Photometric Observations of an SU UMa-type Dwarf Nova VW Coronae Borealis during Outbursts

We report the photometric observations of an SU UMa-type dwarf nova VW CrB during two superoutbursts in 2001 and 2003 and a normal outburst in 2003. Superhumps with a period of 0.07287(1) d were observed during the 2003 superoutburst. The change rate of the superhump period was positive. During the normal outburst, there are some hint of modulation up to a 0.2-mag amplitude. However, any periodicity was not found. The recurrence cycles of the normal outburst and the superoutburst, and the distance were estimated to be > ~50 d, 270-500 d, and 690(+230, -170) pc, respectively. These recurrence cycles are usual values for an SU UMa-type dwarf nova having this superhump period. The superhump period of VW CrB was the longest among those of the SU UMa stars with positive derivatives of the superhump period. The coverage of our observations was, however, not enough, and the variation of the Psh change rate of VW CrB is still unknown. A superhump regrowth and a brightening were seen near the end of the plateau phase. Measuring the deviation of the start timings of the brightening and the superhump regrowth (>2 days in VW CrB) will be a key to reveal the mechanism of these phenomena.Comment: 7 pages, 8 figures, to appear in PAS

Discovery of a Dwarf Nova Breaking the Standard Sequence of Compact Binary Evolution

We revealed that the dwarf nova 1RXS J232953.9+062814 is an SU UMa-type system with a superhump period of 66.774+/-0.010 min. The short period strongly indicates that the orbital period of this object is below the period minimum of cataclysmic variables. The superhump period is 4.04+/-0.02 % longer than the photometric period during quiescence (64.184+/-0.003 min), which is probably associated with the orbital period. Although the standard evolutionary scenario of cataclysmic variables predicts lower mass-transfer rates in systems with shorter orbital periods, we revealed firm evidence of a relatively high mass-transfer rate from its large proper motion and bright apparent magnitude. Its proximity indicates that we have overlooked a number of objects in this new class. With the analogous system of V485 Cen, these objects establish the first subpopulation in hydrogen-rich cataclysmic variables below the period minimum.Comment: 5 pages, 2 figures, PASJ in pres

Discovery of 15-second oscillations in Hubble Space Telescope observations of WZ Sagittae following the 2001 outburst

We report the discovery of 15-s oscillations in ultraviolet observations of WZ Sge obtained with the Hubble Space Telescope approximately one month after the peak of the 2001 outburst. This is the earliest detection of oscillations in WZ Sge following an outburst and the first time that a signal near 15 s has been seen to be dominant. The oscillations are quite strong (amplitude about 5%), but not particularly coherent. In one instance, the oscillation period changed by 0.7 s between successive observations separated by less than 1 hour. We have also found evidence for weaker signals with periods near 6.5 s in some of our data. We discuss the implications of our results for the models that have been proposed to account for the 28-s oscillations seen in quiescence. If the periods of the 15-s oscillations can be identified with the periods of revolution of material rotating about the white dwarf, the mass of the white dwarf must satisfy M_WD > 0.71 M_sun. The corresponding limit for the 6.5-s signals is M_WD > 1.03 M_sun.Comment: accepted for publication in ApJ Letters; 13 pages, 4 postscript figures; new version corrects a few typos and matches version that will appear in ApJ