Orbital and physical parameters of eclipsing binaries from the ASAS catalogue -- III. Two new low-mass systems with rapidly evolving spots

We present the results of our spectroscopic and photometric analysis of two newly discovered low-mass detached eclipsing binaries found in the All-Sky Automated Survey (ASAS) catalogue: ASAS J093814-0104.4 and ASAS J212954-5620.1. Using the GIRAFFE instrument on the 1.9-m Radcliffe telescope at SAAO and the UCLES spectrograph on the 3.9-m Anglo-Australian Telescope, we obtained high-resolution spectra of both objects and derived their radial velocities (RVs) at various orbital phases. The RVs of both objects were measured with the TODCOR technique using synthetic template spectra as references. We also obtained V and I band photometry using the 1.0-m Elizabeth telescope at SAAO and the 0.4-m PROMPT instruments located at the CTIO. The orbital and physical parameters of the systems were derived with PHOEBE and JKTEBOP codes. We compared our results with several sets of widely-used isochrones. Our multi-epoch photometric observations demonstrate that both objects show significant out-of-eclipse modulations, which vary in time. We believe that this effect is caused by stellar spots, which evolve on time scales of tens of days. For this reason, we constructed our models on the basis of photometric observations spanning short time scales (less than a month). Our modeling indicates that (1) ASAS-09 is a main sequence active system with nearly-twin components with masses of M1 = 0.771(33) Msun, M2 = 0.768(21) Msun and radii of R1 = 0.772(12) Rsun and R2 = 0.769(13) Rsun. (2) ASAS-21 is a main sequence active binary with component masses of M1 = 0.833(17) Msun, M2 = 0.703(13) Msun and radii of R1 = 0.845(12) Rsun and R2 = 0.718(17) Rsun. Both systems confirm the characteristic of active low-mass stars, for which the observed radii are larger and the temperatures lower than predicted by evolutionary models. Other parameters agree within errors with the models of main sequence stars.Comment: 15 pages, 7 figures, 7 tables, to appear in A&

Curious Variables Experiment (CURVE). CCD Photometry of QW Serpentis in Superoutburst and Quiescence

We report extensive photometry of the dwarf nova QW Ser throughout its 2003 February superoutburst till quiescence. During the superoutburst the star displayed clear superhumps with a mean period of Psh = 0.07703(4) days. In the quiescence we observed a double humped wave characterized by a period of P=0.07457(2) days. As both periods obey the Stolz-Schoembs relation with a period excess equal to 3.30+/-0.06% the latter period is interpreted as the orbital period of the binary system.Comment: accepted for publication in A&A, 5 pages, 7 figure

Curious Variables Experiment (CURVE). Variable properties of the dwarf nova SS UMi

We report on extensive photometry of the dwarf nova SS Ursae Minoris throughout nine months of 2004. In total, we recorded two superoutbursts and 11 normal outbursts of the star. SS UMi has been known to show frequent superoutbursts with a mean interval of 84.7 days. Our data suggest that the interval between successive superoutbursts lengthened to 197 days, indicating that SS UMi entered a period of untypical behavior manifested by a growth in the quiescent magnitude of the star and a series of frequent, low-amplitude, normal outbursts observed from July to September 2004. The mean superhump period derived for the April 2004 superoutburst of SS UMi is 0.070149(16) days (101.015 min). Combining this value with an earlier orbital period determination, we were able to derive the period excess, which is equal to 3.5 +/- 1.6%, and estimate the mass ratio of the binary system as equal to q=0.16 +/- 0.07. During the entire superoutburst, the period decreased at a rate of $\dot P/P_{\rm sh} = -6.3(1.4) \times 10^{-5}$. However, detailed analysis of the timings of superhump maxima seem to suggest a more complex period change, with a decrease in the period during the first and last stages of the superoutburst but an increase in the middle interval.Comment: 9 pages, 8 figures, in print in Astronomy & Astrophysic