Δχ² map in the DEF gravity parameter space from the simulated 5-yr timing data for three different PSR-BH systems. The left panel corresponds to 10 µs TOA uncertainty, the right panel to 1 µs. Solid lines show the 90% CL limits for PSR-BH systems with different orbital periods: 3 days, 1 day and 8 h. The grey dashed line depicts the limit from the Cassini mission and green dash-dotted line is the current limit from PSR J2222–0137 discussed in Sect. 4 (see Fig. 2).