2003MNRAS.342.1349Y

The prime purposes of this study are to obtain reliable orbital parameters for the Algol-type binary U Coronae Borealis (U CrB) and to explain the evolutionary status of this system. All observations of the primary star's radial velocity are consistent with the value K₁= 58.6±2.0 km/s. Measurements of the radial velocity of the secondary component give K₂=185.2± 5.0 km/s. Using the photometrically determined inclination of 78.7±0.3, the masses of the two stars are therefore deduced to be 4.74±0.28 and 1.46 ±0.06 M_☉for the primary and secondary components, respectively. Using all available observations, we discuss the origin and evolution of the close binary system U CrB. We derive the restrictions concerning masses and period from a general network of calculations of medium mass close binary evolution. Detailed models are calculated within the derived ranges, giving the most likely initial system parameters as {formmu5} and P_i= 1.4 d. It turns out that the interactive evolution up to the present stage has been non-conservative. During its evolution, U CrB has lost about 14 per cent of its initial total mass (ΔM ∼ 1 M_☉) and around 18 per cent of its initial total angular momentum. We also examine the possibility of probing dynamo action in the mass-losing component of U CrB. We point out that, in order to maintain the evolution of U CrB in its later stages, which is presumably driven by stellar `magnetic braking', an efficient mechanism for producing large-scale surface magnetic fields in the donor star is required. We suggest that observed X-ray activity in U CrB may be a good indicator of its evolutionary status and the internal structure of the mass-losing component.