We report the observation of bottom-charmed mesons B-c in 1.8 TeV <p(p)over
bar> collisions using the CDF detector at the Fermilab Tevatron. The B-c m
esons were found through their semileptonic decays, B-c(+/-) --> J/psi l(+/
-)X. A fit to the J/psi l mass distribution yielded 20.4(-5.5)(+6.2) events
from B-c mesons. A test of the null hypothesis, i.e., an attempt to fit th
e data with background alone, was rejected at the level of 4.8 standard dev
iations. By studying the quality of the fit as a function of the assumed B-
c mass, we determined M(B-c) = 6.40 +/- 0.39 (stat.)+/-0.13 (syst) GeV/c(2)
. From the distribution of trilepton intersection points in the plane trans
verse to the beam direction we measured the B-c lifetime to be tau(B-c) = 0
.46(-0.16)(+0.18) (stat)+/-0.03 (syst) ps. We also measured the ratio of pr
oduction cross section times branching fraction for Bc+ --> J/psi l(+) nu r
elative to that for B+ --> J/psi K+ to be
sigma(B-c) X B(B-c --> J/psi l nu)/sigma(B) X B(B-->J/psi K) = 0.132(-0.037
)(+0.041) (stat)+/-0.031 (syst)(-0.020)(+0.032)(lifetime). [S0556-2821(98)0
1421-0].