Four-colour photometry of eclipsing binaries. XXVI A - RY Aqr: A low-mass semidetached system with intrinsic variability The first photoelectric analysis is presented of new uvby light curves(Helt, 1986) of the variable star RY Aquarii. RY Aqr is a double-lined,eclipsing binary consisting of a late A/early F main sequence star andan early K subgiant. The analysis is based on an improved determinationof the dimensions from the light curves combined with the spectroscopicresults from Popper. The analysis is complicated by intrinsicvariability, leading to an accuracy in the radii of about 3 percent andin the masses of 4-5 percent. The hotter star has an unusually low mass(1.27 solar masses) and may be slightly metal deficient (Fe/H = -0.4).Its empirical log g value is large (4.33) so that the star lies close tothe ZAMS. The photometrically best fitting solution leads to a volumefor the K subgiant that is a few percent smaller than its availableRoche lobe volume.
