16244

Mass transfer occurs in binary stars across stellar masses comma and has broad implications for astrophysics. Binary evolution with mass transfer leads to gravitational wave sources comma SNe Ia.s comma low mass X-ray binaries comma sub-dwarf O-B stars comma blue stragglers comma and assuredly new objects in the LSST-era. Mass transfer physics remains uncertain in crucial ways comma one being the relative frequency of stable Roche-lobe overflow (RLOF) and unstable common-envelope (CE) evolution. Theory argues that the fork between RLOF and CE depends on binary mass ratio higher mass ratios go through CE evolution. Blue stragglers are tracers of the evolution of solar-like binary stars with orbital periods of less than 10 comma 000 days. HST discovery of white dwarf (WD) companions showed that most open cluster blue stragglers form via RLOF onto secondary stars near the main-sequence turnoff mass. We propose to study blue stragglers with masses below cluster turnoffs comma specifically the recently discovered blue lurkers in the 4-Gyr open cluster M67. Formed from binaries with higher mass ratios comma these blue lurkers can empirically define the RLOF-CE stability criterion. We will search for hot WD companions in the blue lurker binaries using ACS-SBC FUV photometry comma and determine their formation times from cooling ages. Discovery of WDs would establish that blue lurkers also have predominantly stable RLOF origins. Their larger progenitor mass ratios then provide new guidance for mass-transfer theory comma including stability conditions comma the frequency of CE evolution comma and mass transfer efficiencies. With the cooling age timescales comma we will do detailed modeling of RLOF evolution pathways to the blue lurkers.