– 4 – This leads to a quiescent interval. M ∗ is the mass of the donor during this time of quiescence. The mass of the accretor is approximately equal to M ∗ as well, although the accretor may be slightly more massive than the donor at this point. Thus, during phase 1, the donor has lost an amount of mass M 1 (0) − M ∗ , and the accretor has gained M ∗ − M 2 (2) . Generally, M ∗ − M 2 (2) < M 1 (0) − M ∗ , since the thermal time scales of the two stars are different. The continued growth of the core of the first star, and the accompanying stellar expan- sion, perhaps combined with magnetic braking, eventually cause the donor to fill its Roche lobe again. Mass transfer during the second phase of mass transfer is stable and ends when the donor’s envelope is depleted. Depending on the time elapsed since the start of phase 1, the donor’s core will generally have grown by an amount δ. The first question to answer for KOI-81 and KOI-74 is whether each could be an end state of stable mass transfer. If the answer is “yes”, the donor star would have been filling its Roche lobe until the time mass transfer ceased, and the orbital period is: Given a measured value of the orbital period, Equation (1) and Equation (3) combine to produce a set of possible values for M ∗ . and M$_{c}$. The value of M$_{c}$ should correspond to the present-day mass of the core observed today, and it can be measured through radial velocity or other methods (R2010, van Kerkwijk et al. 2010 [vK2010]). The value of M ∗ represents the mass of the primary, generally as it was during the quiescent interval between phase 1 and phase 2. The black points in the top two panels of Figure 1 show the results for KOI-81 and KOI- 74. The mass of KOI-81b is constrained to lie in a very narrow range: 0 . 215 − 0 . 219 M ⊙ . (Note however that the systematic uncertainty associated with using the functional forms in Equation (1) is larger than this formal range indicates.) The mass M ∗ ranges from roughly 1 . 6 M ⊙ and 2 . 4 M ⊙ . R2019 interpreted light curve features in terms of tidal effects and found the mass of KOI-81b to be 0 . 212 ± 0 . 031 M ⊙ . This is consistent with our mass transfer model. The R2010 value is shown as a cyan triangle centered on a cyan line which delineates the uncertainty limits. vK2010 used Doppler boosting, and measured the mass to be 0 . 3 M ⊙ . vK2010 also used the model of steady mass transfer to predict the mass of KOI-81b, and found a value of 0 . 25 M ⊙ . Their model estimate was marginally higher than ours because they used an expression for the orbital period that did not have any dependence on the P$_{orb}$ = 0 . 372 days [ R$_{d}$ ( M ∗ ,M$_{c}$ ) R ⊙ ]$^{3}$ 2 [ M ⊙ M$_{c}$ ]$^{3}$ 2 . (3) 2.2. Gradual Erosion: KOI-81 and possibly KOI-74