Comparative Study of TESS Photometry and Radial Velocities on Six Early K-type Contact Binaries with Similar Periods around 0.268 Day

According to the definition introduced by Binnendijk (1970), late-type (or W UMa–type) contact binaries (CBs) are classified into two classes: A type and W type. We use “subtype” instead of “type,” as some new literature has used (Li et al. 2020; Lu et al. 2020; Qian et al. 2020; Alton & Stepień 2021; Li et al. 2021; Panchal & Joshi 2021). For A-subtype contact systems, the more massive component has a higher effective temperature compared to the less massive component, while for W-subtype systems, the less massive component has the higher temperature (Yildiz & Doğan 2013; Li et al.