X-ray binaries are stellar systems composed of a neutron star or a black hole that accretes mass from a companion star. The transferred material carries angular momentum and an accretion disc is naturally formed around the compact object. There, the dissipative forces transform the potential energy of the gas into heat. In the process, the gas moves inward as the disc heats up. This mechanism is most efficient in the inner regions, where the temperature reaches ∼ 107 K and most of the energy is released in X-rays.
X-ray binaries are the brightest persistent sources of the X-ray sky. They were discovered in the early days of X-ray astronomy, in the mid-20th century, opening a new avenue to the observational study not only, of the physics of compact objects, but also of accretion processes in the strong gravity regime. They have the advantage of being mostly Galactic objects; they are nearby (compared to active galactic nuclei or quasi-stellar objects) and thus brighter and easier to study. In addition, they present variability on timescales accessible to human lives (from fractions of a second to years). Although X-ray binaries are bright X-ray sources, some of the most important progress in the field has come from multi-wavelength studies.
In this thesis we will analyse three sources catalogued as low-mass X-ray binaries (LMXB), a subclass of X-ray binaries where the companion star is less massive than the Sun. We will use optical spectroscopy as the main tool for studying different properties of the accretion disc, going from scale parameters (radial size and vertical extent) to disc related outflows, which are intimately linked to the accretion process.
The first work presents the analysis of MAXI J1807+132, a transient LMXB discovered in 2017 during an outburst episode. The study is based on X-ray and optical observations spanning over 125 days during the decline of this event. We discuss some important characteristics of MAXI J1807+132 like its distance, the companion star spectral type, the accretion disc size, and the nature of the compact object on the basis of the observed phenomenology and by comparison with other systems.
In the second work we study Aquila X-1, a neutron-star LMXB that shows regular out- bursts about every two years. We used the Gran Telescopio Canarias to carry out an optical spectroscopic follow-up during three consecutive outbursts (2011, 2013, and 2016), obtaining 65 spectra that sample the entire orbital cycle. This allowed us to measure the Kem velocity, which traces the motion of the irradiated face of the companion star. We achieved this by applying the Doppler tomography technique to the narrow lines of the Bowen blend. Based on the mea- sured Kem, and in combination with other dynamical parameters of the system, we carried out a Monte Carlo analysis to empirically determine the accretion disc opening angle for the first time.

Finally, we present high-time resolution optical spectroscopy and imaging of Swift J1357.2– 0933 during its 2017 outburst. Discovered in 2011, this transient system is a black-hole LMXB characterised by showing quasi-periodic optical dips in its light curve. These features have short durations (timescale of minutes) and repeat with increasing quasi-periodicity as the outburst decays. In our work, we used the Gran Telescopio Canarias to spectroscopically resolve the dips for the first time. We observe blue-shifted absorptions during these dips, that demonstrate that these features are associated with cold (low ionisation) winds. We discuss the properties of this outflow and the possible wind launching mechanism.
This thesis shows the potential of optical spectroscopic studies complemented by X-ray data to constrain the nature of X-ray binaries as well as the properties of the X-ray heated discs and associated outflows. The three works presented here have been published in Monthly Notices of the Royal Astronomical Society between 2018 and 2019.