  Five years of Rossi X-ray Timing Explorer (RXTE) observations of the Galactic
black-hole candidates 1E 1740.7-2942 and GRS 1758-258 show a periodic
modulation with amplitude 3-4% in each source at 12.73 +/- 0.05 dy and 18.45
+/- 0.10 dy, respectively. We interpret the modulations as orbital, suggesting
that the objects have red-giant companions. Combining the RXTE data with
earlier data (Zhang, Harmon & Liang 1997) from the Burst and Transient Source
Experiment on the Compton Gamma-Ray Observatory, we find a long period or
quasi-period of about 600 dy in 1E 1740.7-2942, and a suggestion of a similar
600-dy period in GRS 1758-258. These timescales are longer than any yet found
for either precessing systems like Her X-1 and SS 433 or binaries like LMC X-3
and Cyg X-1 with more irregular long periods.
