The first photometric and spectroscopic study of contact binary V2840 Cygni.


 The first photometric, spectroscopic, and period variation studies of neglected short-period eclipsing binary V2840 Cygni are presented. High Mass Ratio Contact Binaries, especially those in the weak-contact configuration are vital while probing into the evolutionary models of CBs using stellar parameters. The photometric solutions reveal the weak-contact nature of V2840 Cygni with a high mass ratio ($\sim1.36$), motivating us to investigate the nature of such binaries. The period variation study of V2840 Cygni spanning for 15 years shows a secular period decrease at a rate of \begin{math}\sim5.5\times10^{-7}\end{math}d/yr indicating mass transfer between the components. The superimposed cyclic variation provides a basic understanding of the possible third body (\emph{P$_3$}$\sim8$ yr, \emph{m$_3$}$\sim0.51$M$_\odot$). Following the derived parameters, the evolution of the system is discussed based on Thermal Relaxation Oscillation (TRO) model. It is found that V2840 Cygni falls in a special category of HMRCBs, that validates TRO. To characterise the nature of HMRCBs, a catalog of 114 CBs with high mass ratios has been compiled along with their derived parameters from the literature. For all the HMRCBs in the study, a possible correlation between their contact configuration and observed period variations for relative log\emph{J$_{rel}$} is discussed. The spectroscopic study of V2840 Cygni provides evidence of the presence of magnetic activity in the system and the existence of ongoing mass transfer which is additionally deduced from the period variation study. The LAMOST spectra of 30 HMRCBs are collected to interpret the stellar magnetic activity in such systems.