  In recent years, thanks to the continuous surveys performed by INTEGRAL and
Swift satellites, our knowledge of the hard X-ray/soft gamma-ray sky has
greatly improved. As a result it is now populated with about 2000 sources, both
Galactic and extra-galactic, mainly discovered by IBIS and BAT instruments.
Many different follow-up campaigns have been successfully performed by using a
multi-wavelength approach, shedding light on the nature of a number of these
new hard X-ray sources. However, a fraction are still of a unidentified nature.
This is mainly due to the lack of lower energy observations, which usually
deliver a better constrained position for the sources, and the unavailability
of the key observational properties, needed to obtain a proper physical
characterization. Here we report on the classification of two poorly studied
Galactic X-ray transients IGR J20155+3827 and Swift J1713.4-4219, for which the
combination of new and/or archival X-ray and Optical/NIR observations have
allowed us to pinpoint their nature. In particular, thanks to XMM\Newton
archival data together with new optical spectroscopic and archival Optical/NIR
photometric observations, we have been able to classify IGR J20155+3827 as a
distant HMXB. The new INTEGRAL and Swift data collected during the 2019 X-ray
outburst of Swift J1713.4-4219, in combination with the archival optical/NIR
observations, suggest a LMXB classification for this source.

High Energy Astrophysical Phenomena