Recent statistical studies prove that the percentage of RR Lyrae pulsators that are located in binaries or multiple stellar systems is considerably lower than might be expected. This can be better understood from an in-depth analysis of individual candidates. We investigate in detail the light time effect of the most probable binary candidate TU UMa. This is complicated because the pulsation period shows secular variation. We model possible light time effect of TU UMa using a new code applied on previously available and newly determined maxima timings to confirm binarity and refine parameters of the orbit of the RRab component in the binary system. The binary hypothesis is also tested using radial velocity measurements. We used new approach to determine brightness maxima timings based on template fitting. This can also be used on sparse or scattered data. This approach was successfully applied on measurements from different sources. To determine the orbital parameters of the double star TU UMa, we developed a new code to analyse light time effect that also includes secular variation in the pulsation period. Its usability was successfully tested on CL Aur, an eclipsing binary with mass-transfer in a triple system that shows similar changes in the O-C diagram. Since orbital motion would cause systematic shifts in mean radial velocities (dominated by pulsations), we computed and compared our model with centre-of-mass velocities. They were determined using high-quality templates of radial velocity curves of RRab stars. Maxima timings adopted from the GEOS database (168) together with those newly determined from sky surveys and new measurements (85) were used to construct an O-C diagram spanning almost five proposed orbital cycles. This data set is three times larger than data sets used by previous authors. Modelling of the O-C dependence resulted in 23.3-year orbital period, which translates into a minimum mass of the second component of about 0.33Ms. Secular changes in the pulsation period of TU UMa over the whole O-C diagram were satisfactorily approximated by a parabolic trend with a rate of -2.2ms/yr. To confirm binarity, we used radial velocity measurements from nine independent sources. Although our results are convincing, additional long-term monitoring is necessary to unambiguously confirm the binarity of TU UMa.
This is the second in a series of papers presenting a new calculation of the mass of the Galaxy based on radial velocities and distances for a sample of faint 16<B<21.3 field blue horizontal-branch (BHB) stars. We present accurate BV CCD photometry and spectra for 142 candidate A-type stars selected from ubjr photometry of UK Schmidt telescope plates in six high-Galactic-latitude fields. Classification of these candidates produces a sample of 60 BHB stars at distances of 11-52kpc from the Sun (mean 28kpc), with heliocentric line-of-sight velocities accurate to 15km/s, and distance errors <10 per cent. We provide a summary table listing coordinates and velocities of these stars.
We report broadband UBV and/or BVR_C_I_C_ CCD photometry for a total of 1857 stars in the thick-disk and halo populations of the Galaxy. The majority of our targets were selected as candidate field horizontal-branch or other A-type stars (FHB/A, N=576), or candidate low-metallicity stars (N=1221), from the HK and Hamburg/ESO objective-prism surveys. Similar data for a small number of additional stars from other samples are also reported.
A color-magnitude diagram of the globular cluster NGC 6426 has been constructed from photometry in the V and I passbands. The cluster has the steep giant branch indicative of a very metal-poor composition. Following the technique of Sarajedini (1994AJ....107..618S), we derive [Fe/H]=-2.31+/-0.10 and E(V-I)=0.54+/-0.03 [E(B-V)=0.40+/-0.02] from a comparison with the giant branches of other globular clusters. These values are very close to the ones obtained from photometry of the cluster's integrated light, and the weighted means of our results and the previous ones are [Fe/H]=-2.28+/-0.09 and E(B-V)=0.41+/-0.02. The mean V magnitude of the horizontal branch (HB) at the instability strip is 18.12+/-0.05, from which we estimate that NGC 6426 lies 17.4kpc from the Sun, 11.5kpc from the galactic center, and 4.9kpc from the galactic plane. NGC 6426 resembles NGC 5466 and M68 in both [Fe/H] and HB morphology. The HB of NGC 6426 overlaps the one in M15 from the red HB to M15's gap in the blue HB, but there is no evidence of significant numbers of faint blue HB stars in NGC 6426 that are analogous to the ones constituting the "extended HB" in M15.
