We present Hubble Space Telescope data of the low-reddening Sagittarius
window in the Galactic bulge. The Sagittarius Window Eclipsing Extrasolar
Planet Search field (3'x3'), together with three more Advanced Camera for
Surveys and eight Wide Field Camera 3 fields, were observed in the F606W and
F814W filters, approximately every two weeks for two years, with the principal
aim of detecting a hidden population of isolated black holes and neutron stars
through astrometric microlensing. Proper motions were measured with an accuracy
of ~0.1 mas/yr (~4 km/s) at F606W~25.5 mag, and better than ~0.5 mas/yr (20
km/s) at F606W~28 mag, in both axes. Proper-motion measurements allowed us to
separate disk and bulge stars and obtain a clean bulge color-magnitude diagram.
We then identified for the first time a white dwarf (WD) cooling sequence in
the Galactic bulge, together with a dozen candidate extreme horizontal branch
stars. The comparison between theory and observations shows that a substantial
fraction of the WDs (30%) are systematically redder than the cooling tracks for
CO-core H-rich and He-rich envelope WDs. This evidence would suggest the
presence of a significant number of low-mass WDs and WD - main sequence
binaries in the bulge. This hypothesis is further supported by the finding of
two dwarf novae in outburst, two short-period (P < 1 d) ellipsoidal variables,
and a few candidate cataclysmic variables in the same field.Comment: 9 pages, 5 figures, accepted for publication on Ap
