We have used 605 days of photometric data from the Kepler spacecraft to study KIC 6614501 , a close binary system with an orbital period of 0.15749747 ( 25 ) days ( 3.779939 hours ) , that consists of a low-mass subdwarf B ( sdB ) star and a white dwarf .
As seen in many other similar systems , the gravitational field of the white dwarf produces an ellipsoidal deformation of the sdB which appears in the light curve as a modulation at two times the orbital frequency .
The ellipsoidal deformation of the sdB implies that the system has a maximum inclination of \sim 40 degrees , with i \approx 20 ^ { \circ } being the most likely .
The orbital radial velocity of the sdB star is high enough to produce a Doppler beaming effect with an amplitude of 432 \pm 5 ppm , clearly visible in the folded light curve .
The photometric amplitude that we obtain , K _ { 1 } = 85.8 ~ { } km / s , is \sim 12 per cent less than the spectroscopic RV amplitude of 97.2 \pm 2.0 ~ { } km / s .
The discrepancy is due to the photometric contamination from a close object at about 5 arcsec North West of KIC 6614501 , which is difficult to remove .
The atmospheric parameters of the sdB star , T _ { eff } = 23 ~ { } 700 \pm 500 ~ { } K and \log g = 5.70 \pm 0.10 , imply that it is a rare object below the Extreme Horizontal Branch ( EHB ) , similar to HD 188112 ( ) .
The comparison with different evolutionary tracks suggests a mass between \sim 0.18 and \sim 0.25 { \mathrm { M } } _ { \odot } , too low to sustain core helium burning .
If the mass was close to 0.18-0.19 { \mathrm { M } } _ { \odot } , the star could be already on the final He-core WD cooling track .
A higher mass , up to \sim 0.25 { \mathrm { M } } _ { \odot } , would be compatible with a He-core WD progenitor undergoing a cooling phase in a H-shell flash loop .
A third possibility , with a mass between \sim 0.32 and \sim 0.40 { \mathrm { M } } _ { \odot } , can not be excluded and would imply that the sdB is a “ normal ” ( but with an unusually low mass ) EHB star burning He in its core .
In all these different scenarios the system is expected to merge in less than 3.1 Gyr due to gravitational wave radiation .