We present a light curve analysis and radial velocity study of KOI-74 , an eclipsing A star + white dwarf binary with a 5.2 day orbit .
Aside from new spectroscopy covering the orbit of the system , we used 212 days of publicly available Kepler observations and present the first complete light curve fitting to these data , modelling the eclipses and transits , ellipsoidal modulation , reflection , and Doppler beaming .
Markov Chain Monte Carlo simulations are used to determine the system parameters and uncertainty estimates .
Our results are in agreement with earlier studies , except that we find an inclination of 87.0 \pm 0.4 ^ { \circ } , which is significantly lower than the previously published value .
The altered inclination leads to different values for the relative radii of the two stars and therefore also the mass ratio deduced from the ellipsoidal modulations seen in this system .
We find that the mass ratio derived from the radial velocity amplitude ( q = 0.104 \pm 0.004 ) disagrees with that derived from the ellipsoidal modulation ( q = 0.052 \pm 0.004 assuming corotation ) .
This was found before , but with our smaller inclination , the discrepancy is even larger than previously reported .
Accounting for the rapid rotation of the A-star , instead of assuming corotation with the binary orbit , is found to increase the discrepancy even further by lowering the mass ratio to q = 0.047 \pm 0.004 .
These results indicate that one has to be extremely careful in using the amplitude of an ellipsoidal modulation signal in a close binary to determine the mass ratio , when a proof of corotation is not firmly established .
The same problem could arise whenever an ellipsoidal modulation amplitude is used to derive the mass of a planet orbiting a host star that is not in corotation with the planet ’ s orbit .

The radial velocities that can be inferred from the detected Doppler beaming in the light curve are found to be in agreement with our spectroscopic radial velocity determination .
We also report the first measurement of Rømer delay in a light curve of a compact binary .
This delay amounts to -56 \pm 17 s and is consistent with the mass ratio derived from the radial velocity amplitude .
The firm establishment of this mass ratio at q = 0.104 \pm 0.004 leaves little doubt that the companion of KOI-74 is a low mass white dwarf .