An eruptive BL Lacertae object with a high redshift, 0846 + 51 W1

The optical counterpart of the radio source 0846 + 51W1 was observed to brighten from V of about 19.5 mag to V = 15.7 mag in less than one month. Additional optical and radio observations are reported. They show: (1) at maximum light outburst, the continuum is featureless and the object has all the radio and optical characteristics of a BL Lacertae object (except polarization, which is unknown); (2) the normal light level V is between 19 and 21 mag; (3) throughout its phases, the optical B-V and U-B colors are appreciably redder than any quasar; (4) the spectrum at minimum light has emission lines that yield a redshift of z = 1.860, which is the highest redshift so far found for a BL Lacertae object; (5) the BL Lacertae object lies only 12 arcsec south of the southern member of a pair of interacting spirals; (6) one possible emission line is not identified in the z = 1.86 redshift system. This line could be part of a second emission-line redshift system, but this would require confirmation by additional observation.