We report on a recent event in which , after more than a decade of slowly fading , the visual brightness of the massive young binary Z CMa suddenly started to rise by about 1 magnitude in December 1999 , followed by a rapid decline to its previous brightness over the next six months .
This behaviour is similar to that exhibited by this system around its eruption in February 1987 .
A comparison of the intrinsic luminosities of the system with recent evolutionary calculations shows that Z CMa may consist of a 16 M _ { \odot } B0 IIIe primary star and a \sim 3 M _ { \odot } FUor secondary with a common age of \sim 3 \times 10 ^ { 5 } yr. We also compare new high-resolution spectra obtained in Jan. and Feb. 2000 , during the recent rise in brightness , with archive data from 1991 and 1996 .
The spectra are rich in emission lines , which originate from the envelope of the early B-type primary star .
The strength of these emission lines increased strongly with the brightness of Z CMa .
We interpret the collected spectral data in terms of an accretion disc with atmosphere around the Herbig B0e component of Z CMa , which has expanded during the outbursts of 1987 and 2000 .
A high resolution profile of the 6300 Å [ O i ] emission line , obtained by us in March 2002 shows an increase in flux and a prominent blue shoulder to the feature extending to \sim - 700 km s ^ { -1 } , which was much fainter in the pre-outburst spectra .
We propose that this change in profile is a result of a strong change in the collimation of a jet , as a result of the outburst at the start of this century .