ῥά
From LSJ
Κύριε, βοήθησον τὸν δοῦλον σου Νῖλον κτλ. → Lord, help your slave Nilos ... (mosaic inscription from 4th-cent. church in the Negev)
English (LSJ)
[ᾰ], enclit. Particle, Ep. for ἄρα (q.v.), freq. in Hom.and Pi.; less freq. in Trag. (lyr.),
A ἦ ῥα A.Pers.633, S.Aj.172; ἤ ῥα ib.177, B. 18.33.—This and κα are the only monosyll. Particles not ending in ε which allow elision.