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We used a combination of deep optical and Halpha images of the Galacticglobular cluster NGC 6440, acquired with the Hubble Space Telescope, toidentify the optical counterpart to the accreting millisecond X-ray pulsar SAXJ1748.9-2021during quiescence. A strong Halpha emission has been detected froma main sequence star (hereafter COM-SAX J1748.9-2021) located at only 0.15"from the nominal position of the X-ray source. The position of the star alsoagrees with the optical counterpart found by Verbunt et al. (2000) during anoutburst. We propose this star as the most likely optical counterpart to thebinary system. By direct comparison with isochrones, we estimated that COM-SAXJ1748.9-2021 has a mass of 0.70 Msun - 0.83 Msun, a radius of 0.88 pm 0.02 Rsunand a superficial temperature of 5250pm80 K. These parameters combined with theorbital characteristics of the binary suggest that the system is observed at avery low inclination angle (~8 deg -14 deg) and that the star is filling oreven overflowing its Roche Lobe. This, together with the equivalent width ofthe Halpha emission (~20 Ang), suggest possible on-going mass transfer. Thepossibile presence of such a on-going mass transfer during a quiescence statealso suggests that the radio pulsar is not active yet and thus this system,despite its similarity with the class of redback millisecond pulsars, is not atransitional millisecond pulsar.

The Optical Counterpart to the Accreting Millisecond X-ray Pulsar SAX J1748.9-2021 in the Globular Cluster NGC 6440