  We study the periodic Schr\"odinger-Korteweg de Vries system. We describe the
two-parametetric family of $2T$ periodic traveling waves of dnoidal type. The
main objective of the paper is to establish their spectral stability with
respect to co-periodic perturbations. In the limit $T\to \infty$, we recover
the results of Albert-Angulo, for the stability of the soliton solutions of
this system on the real line.
