Orbital and superorbital periods of 1E 1740.7-2942 and GRS 1758-258

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 an amplitude of 3%-4% in each source at 12.73 +/- 0.05 and 18.45 +/- 0.10 days, 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) from the Burst and Transient Source Experiment on the Compton Gamma Ray Observatory, we find a long period or quasi-period of about 600 days in 1E 1740.7-2942 and a suggestion of a similar 600 day period in GRS 1758-258. These timescales are longer than any yet found for either precessing systems like Hercules X-1 and SS 433 or binaries like LMC X-3 and Cygnus X-1 with more irregular long periods.