1999AJ....118.2442W

The epochs of maximum light (EMLs) in the light curve of TU UMa, a halo RR Lyrae variable star, depart from a linear ephemeris over the 85 yr interval that the star has been under scrutiny. We have supplemented the list of EMLs assembled by Saha & White with new photoelectric (photomultiplier tube or CCD) measurements made during the period 1990-1999; in addition, we have collected other recent EMLs from the literature. There are now 83 EMLs in the list, of which 35 are reported for the first time here. We analyze these data, combining a quadratic pulsation ephemeris with an orbital timing model, as previously suggested by Szeidl et al. and by Saha & White. The favored model relies exclusively on photoelectric measurements of EMLs, and it yields residuals (2.4 minutes rms) that are comparable to the precision of measurement. We conclude that TU UMa is a member of a binary star system with a period of approximately 23 yr. This conclusion is supported by available high-quality center-of-mass radial velocity data. The inferred orbit is highly eccentric, e~0.79, but not extremely so. The minimum mass of the companion object in the favored model is ~0.4 M_☉. The minimum projected size of TU UMa's orbital major axis is 10 mas. The binary system is presently near its widest orbital and angular separation. TU UMa is the only RR Lyrae star that can be considered with high probability to be a member of a relatively short-period binary and as such is the only RR Lyrae star for which there is a prospect for direct measurement of the mass.