Search for spontaneous R-parity violation at root s=183 GeV and 189 GeV

Searches for spontaneous $R$-parity violating signals at $\sqrt{s}=183$\,GeV and \mbox{$\sqrt{s}=189$\,GeV} have been performed using 1997 and 1998 DELPHI data, under the assumption of $R$-parity breaking in the third lepton family. The expected topology for the decay of a pair of charginos into two acoplanar taus plus missing energy was investigated and no evidence for a signal was found. The results were used to derive a limit on the chargino mass and to constrain the allowed domains of the MSSM parameter sp.