Absolute properties of RU Cnc revisited: an active RS CVn-type eclipsing binary with red giant branch and main-sequence components

We present observations and analysis of an RS CVn-type double-lined eclipsing binary system, RU Cnc. The system has been observed for over a century. High-quality long-cadence observations, newly obtained from the Kepler K2 C5 and C18 campaigns, and two radial velocity curves were combined and analysed simultaneously, assuming a multispot model. The masses, radii and luminosities of the component stars have been precisely obtained as M_{c} = 1.386± 0.044 M_{⊙ }, M_{h} = 1.437 ± 0.046 M_{⊙}, R_{h} = 2.39± 0.07 R_{⊙}, R_{c} = 5.02 ± 0.08 R_{⊙}, L_{h} = 11.4± 1.2 L_{⊙} and L_{c} = 12.0 ± 1.0 L_{⊙}, with a separation of a = 27.914 ± 0.016 R_{⊙}. The distance of the system is determined to be 380± 57 pc, which is consistent with the Gaia Data Release 2 result. Long-term detailed period variation analysis of the system indicates a period decrease of 7.9 × 10^-7 d yr^-1. The results suggest that the cooler component is on the red giant branch (RGB) and the hotter component is still on the main sequence.