We present Hubble Space Telescope data of the low-reddening Sagittarius window in the Galactic bulge .
The Sagittarius Window Eclipsing Extrasolar Planet Search field ( \sim 3′ \times 3′ ) , together with three more Advanced Camera for Surveys and eight Wide Field Camera 3 fields , were observed in the F 606 W and F 814 W 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 \approx 0.1 mas yr ^ { -1 } ( \approx 4 km s ^ { -1 } ) at F 606 W \approx 25.5 mag , and better than \approx 0.5 mas yr ^ { -1 } ( \approx 20 km s ^ { -1 } ) at F 606 W \approx 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 ( \approx 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 \lesssim 1 d ) ellipsoidal variables , and a few candidate cataclysmic variables in the same field .