Are the red dwarfs in cataclysmic variables main-sequence stars?

TL;DR

This paper investigates whether the secondary stars in cataclysmic variables are main-sequence stars, finding that short-period systems have secondaries similar to solar-abundance main sequence stars, while longer-period systems show signs of evolution or altered spectral types.

Contribution

The study provides observational evidence that secondaries in short-period cataclysmic variables are main-sequence stars and explores the effects of evolution and mass transfer in longer-period systems.

Findings

Secondaries in P < 3 hr systems are close to solar-abundance main sequence stars.

Longer-period systems often have later spectral types than main sequence stars.

High mass transfer rates and evolved donors are common in these systems.

Abstract

We show that the secondaries in short-period cataclysmic variables with orbital periods $P < 3$ hr are close to the solar-abundance main sequence defined by single field stars. In cataclysmic variables with $P > 3$ hr, the earliest spectral types at a given period correspond to main sequence stars, while the majority of secondaries have later spectral types. Possible causes are nuclear evolution prior to mass transfer and lack of thermal equilibrium due to mass transfer. A comparison with evolutionary sequences obtained with up--to--date stellar models implies unusually high transfer rates and a large fraction of systems with evolved donors. There is no evidence for a secondary of low metallicity in any of the well-studied cataclysmic variables.