A laser consisting of two independently-pumped resonators can exhibit mode bifurcations that evolve out of the exceptional points (EPs) of the linear system at threshold. The EPs are non-Hermitian degeneracies occurring at the parity/time-reversal (PT ) symmetry breaking points of the threshold system. Above threshold, the EPs become bifurcations of the nonlinear zero-detuned laser modes, which can be most easily observed by making the gain saturation intensities in the two resonators substantially different. Small pump variations can then switch abruptly between different laser behaviors, e.g. between below-threshold and PT -broken single-mode operation.Introduction.-Non-Hermitian effects have been the subject of strong and sustained attention in optics and related fields, driven largely by the rise of parity/timereversal (PT ) symmetric photonics [1-3]. Following early proposals to utilize PT symmetry for unidirectional invisibility cloaks [4,5], laser-absorbers [6-8], etc., much recent progress has been based on the complex interactions between PT symmetry breaking and nonlinearity [9][10][11][12][13].Recent prominent experiments exploring this direction have demonstrated efficient optical isolation [14-16], robust wireless power transfer [17], and the stabilization of single-mode lasing in microcavity lasers [18][19][20][21].Lasers provide a compelling setting for such investigations, as they are intrinsically both non-Hermitian (due to gain and outcoupling loss), and nonlinear when operating above threshold (due to gain saturation). Although these features have long been known, the recent interest in non-Hermitian physics has provided fresh inspiration for devising lasers with novel characteristics. Several authors have drawn special attention to the peculiar effects of exceptional points (EPs)-points in parameter space where a non-Hermitian Hamiltonian becomes defective and two (or more) eigenvectors coalesce [22,23]. PT symmetry provides a convenient, though not exclusive, way to generate EPs: whenever PT symmetry spontaneously breaks, there is an EP at the transition point [24]. The presence of an EP among the non-Hermitian eigenmodes of a laser has been shown to give rise to pumpinduced suppression and revival of lasing [26][27][28], though these works regarded the EP as "accidental", not arising from PT symmetry breaking. EPs have also been shown to promote single-mode operation in dark state lasers [29,30] and PT -symmetric lasers [18,31].In this paper, we show that EPs associated with PT symmetry breaking in a laser at threshold can generate bifurcations in the nonlinear laser modes above threshold. The bifurcations appear as discontinuities in the laser's I-V curve (i.e., the dependence of output