Variation of discrete spectra of non-negative operators in Krein spaces

We study the variation of the discrete spectrum of a bounded non-negative operator in a Krein space under a non-negative Schatten class perturbation of order p. It turns out that there exist so-called extended enumerations of discrete eigenvalues of the unperturbed and perturbed operator, respectively, whose difference is an ℓp-sequence. This result is a Krein space version of a theorem by T. Kato for selfadjoint operators in Hilbert spaces.