Spot modelling and elements of the RS CVn eclipsing binary WY Cancri

Results of analysis of photoelectric observations of the RS CVn eclipsing binary WY Cancri in the standard passbands ofUBV during 1973-74, 1976-79 and inUBVRI during 1984-86 are reported. A preliminary analysis of the eclipses suggested the primary eclipse to be transit. A study of the percentage contribution of the distortion wave amplitudes in all the colours with respect to the luminosities of both components, showed the hotter component to be the source of the distortion wave. The clean (wave removed) light curves of different epochs have not merged, suggesting residual effects of spot activity. The reason for this is attributed to the presence of either (1) polar spots or (2) small spots uniformly distributed all over the surface of the hotter component. This additional variation is found to have a periodicity of about 50 years or more. The distortion waves in yellow colour are modelled according to Budding’s (1977) method. For getting the best fit of the observations and theory, it was found necessary to assume three or four spots on the surface of the hot component. Out of these four spot groups, three are found to have direct motion with migration periods of 1.01, 1.01 and 2.51 years while the fourth one has a retrograde motion with a migration period of 3.01 years. From these periods and the latitudes of the spots derived from the model a co-rotating latitude of 4ℴ is obtained. The temperatures of these spots are found to be lower than that of the photosphere by about 700ℴK to 800ℴK. Assuming the light curve of 1985-86, which is the brightest of all the observed seasons, to be least affected by the spots, the light curves of the other seasons are all brought up to the quadrature level of this season by applying suitable corrections. The merged curves in theUBVRI colours are analysed for the elements by the Wilson-Devinney method. This analysis yielded the following absolute elements: $$\begin{gathered} m_h = 0.86 \pm 0.03{\text{ }}M_ \odot \hfill \\ m_h = 0.51 \pm 0.03{\text{ }}M_ \odot \hfill \\ R_h = 0.99 \pm 0.02{\text{ }}R_ \odot \hfill \\ R_c = 0.65 \pm 0.02{\text{ }}R_ \odot \hfill \\ T_h = 5520^o K \pm 100^o K \hfill \\ T_c = 3740^o K \pm 20^o K \hfill \\ M_{h(bol)} = 4\mathop .\limits^m 96 \pm 0.10 \hfill \\ M_{c(bol)} = 7\mathop .\limits^m 58 \pm 0.15 \hfill \\ Spectral type hotter component = G5 \pm 1 \hfill \\ cooler component = K9 \pm 1 \hfill \\ \end{gathered} $$ For a mass ratio of 0.506 and with the derived fractional radii rh = 0.241 andr c = 0.157, both the components are found to be within their Roche lobes. Hence we have classified WY Cnc as a detached system. From their positions on the HR diagram it is concluded that both the components of WY Cnc belong to the main sequence.