Optical/X-ray luminosity relation for LMXBs. The blue line and shaded region show the empirical correlation and its 1σ uncertainty for hard-state BH XRBs, while the green line and shaded region show those for hard-state NS XRBs (Russell et al. 2006). The star symbol marks EP J175257.3–351923 at an assumed distance of 8 kpc, with a de-reddened V-band luminosity near the outburst peak of ≲4 × 10³⁴ erg s⁻¹ and a quasi-simultaneous 2–10 keV X-ray luminosity of ∼3 × 10³⁶ erg s⁻¹.