  We study critical points of a one-parameter family of functionals arising in
combustion models. The problems we consider converge, for infinitesimal values
of the parameter, to Bernoulli's free boundary problem, also known as one-phase
problem. We prove a $C^{1,\alpha}$ estimates for the "interfaces" (level sets
separating the burnt and unburnt regions). As a byproduct, we obtain the
one-dimensional symmetry of minimizers in the whole $\mathbb{R}^N$, for $N \leq
4$, answering positively a conjecture of Fern\'andez-Real and Ros-Oton. Our
results are to Bernoulli's free boundary problem what Savin's results for the
Allen-Cahn equation are to minimal surfaces.
