Abstract: Aims. We study a sample of HB emission line sources at z - 0.9 to identify the star-forming galaxies sample and characterise them in terms of line luminosity, stellar mass, star formation rate, and morphology. The final aim is to obtain the HB luminosity function of the star-forming galaxies at this redshift. Methods. We used the red tunable filter of the instrument Optical System for Imaging low Resolution Integrated Spectroscopy (OSIRIS) at Gran Telescopio de Canarias to obtain the pseudo spectra of emission line sources in the OTELO field. From these pseudo spectra, we identified the objects with HB emission. As the resolution of the pseudo spectra allowed us to separate HB from [O iii], we were able to derive the HB flux without contamination from its adjacent line. Using data from the extended OTELO catalogue, we discriminated AGNs and studied the star formation rate, the stellar mass, and the morphology of the star-forming galaxies. Results. We find that our sample is located on the main sequence of star-forming galaxies. The sources are morphologically classified, mostly as disc-like galaxies (76%), and 90% of the sample are low-mass galaxies (M < 1010 M). The low-mass star-forming galaxies at z - 0.9 that were detected by OTELO present similar properties as low-mass star-forming galaxies in the local universe, suggesting that these kinds of objects do not have a favorite epoch of formation and star formation enhancement from z - 1 to now. Our sample of 40 H star-forming galaxies include the faintest H emitters detected so far. This allows us to constrain the faint end of the luminosity function for the H line alone with a minimum luminosity of log L = 39 erg s1, which is a hundred times fainter than previous surveys. The dust-corrected OSIRIS Tunable Emission Line Object survey (OTELO) H? luminosity function established the faint-end slope as = ?1.36 ± 0.15. We increased the scope of the analysis to the bright end by adding ancillary data from the literature, which was not dust-corrected in this case. The obtained slope for this extended luminosity function is = 1.43 ± 0.12.

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