OUTBURST OF THE ILLUMINATING STAR OF THE BIPOLAR REFLECTION NEBULA RNO-138

Direct images taken in 1988 and 1993 and spectra of the bipolar reflec tion nebula RNO 138 are presented. The images show that the illuminati ng star of the nebula, RNO 138S, for the first time visible in the 199 3 image, has brightened by greater-than-or-equal-to -1.2 mag sometime between 1988 and 1993. The spectrum presents absorption lines, includi ng strong LiI 6708, weak Halpha emission with a possible blueshifted P Cygni absorption and no emission lines. At present, an EXor-type outb urst can be excluded as an explanation for the brightening of RNO 138S and the data are not completely compatible with a FUor-type outburst.