Revisiting atmospheric Roche lobe overflow in symbiotic binaries

Classical binary evolution models predict dynamically unstable mass transfer in symbiotic stars with high mass ratios, leading to a common envelope. However, many observed S-type symbiotic systems show long-lived interaction, suggesting that an additional stabilizing mechanism may be at work. We investigate whether atmospheric Roche-lobe overflow can prolong the mass-transfer phase and help reconcile theory with observations. We implement the Rapid Unified Mass Transfer framework in MESA and compute a grid of white-dwarf--giant binaries covering a wide range of donor masses, mass ratios, and orbital periods. We then compare the resulting lifetimes and evolutionary tracks with well-constrained Galactic S-type symbiotic systems. For convective giant donors, our models recover stable mass transfer up to q 1.5, while atmospheric overflow strongly extends the symbiotic phase. RGB and early-AGB systems with q 1.5 can remain interacting for up to 10 6 yr at 10-9,M, yr-1, much longer than the commonly assumed about 10 cubed yr pre-common-envelope lifetime. In these systems, the orbit shrinks mildly and may re-expand after mass-ratio reversal. Systems with higher mass ratios still evolve toward a common envelope, but even for q 2 -- 4 the symbiotic phase can last 10 4 -- 10 5 yr. The synthetic distribution in the orbital-period--mass-ratio plane and individual evolutionary tracks are broadly consistent with observed S-type symbiotic binaries, including recurrent novae. The RUMT framework, which incorporates atmospheric RLOF, provides an explanation for the long-term stability of many symbiotic binaries and may account for their high observed occurrence rate.