The eclipsing binary system 2M 1938+4603 consists of a pulsating hot subdwarf B star and a cool M dwarf companion in an effectively circular three–hour orbit .
The light curve shows both primary and secondary eclipses , along with a strong reflection effect from the cool companion .
Here we present constraints on the component masses and eccentricity derived from the Rømer delay of the secondary eclipse .
Using six months of publicly-available Kepler photometry obtained in Short Cadence mode , we fit model profiles to the primary and secondary eclipses to measure their centroid values .
We find that the secondary eclipse arrives on average 2.06 \pm 0.12 s after the midpoint between primary eclipses .
Under the assumption of a circular orbit , we calculate from this time delay a mass ratio of q = 0.2691 \pm 0.0018 and individual masses of M _ { sd } = 0.372 \pm 0.024 M _ { \sun } and M _ { c } = 0.1002 \pm 0.0065 M _ { \sun } for the sdB and M dwarf , respectively .
These results differ slightly from those of a previously-published light curve modeling solution ; this difference , however , may be reconciled with a very small eccentricity , e \cos \omega \approx 0.00004 .
We also report an orbital period decrease of \dot { P } = ( -1.23 \pm 0.07 ) \times 10 ^ { -10 } .