Context : The physical processes driving the evolution of star formation ( SF ) in galaxies over cosmic time still present many open questions .
Recent galaxy surveys allow now to study these processes in great detail at intermediate redshift ( 0 \leq z \leq 0.5 ) .

Aims : We build a complete sample of star-forming galaxies and analyze their properties , reaching systems with low stellar masses and low star formation rates ( SFRs ) at intermediate-to-low redshift .

Methods : We use data from the SHARDS multiband survey in the GOODS-North field .
Its depth ( up to magnitude \langle m _ { 3 \sigma } \rangle \sim 26.5 ) and its spectro-photometric resolution ( R \sim 50 ) provides us with an ideal dataset to search for emission line galaxies ( ELGs ) .
We develop a new algorithm to identify low-redshift ( z ¡0.36 ) ELGs by detecting the [ OIII ] 5007 and H \alpha emission lines simultaneously .
We fit the spectral energy distribution ( SED ) of the selected sample , using a model with two single stellar populations .

Results : We find 160 star-forming galaxies for which we derive equivalent widths ( EWs ) and absolute fluxes of both emission lines .
We detect EWs as low as 12 Å , with median values for the sample of \sim 35 Å in [ OIII ] 5007 and \sim 56 Å in H \alpha , respectively .
Results from the SED fitting show a young stellar population with low median metallicity ( 36 % of the solar value ) and extinction ( A _ { V } \sim 0.37 ) , with median galaxy stellar mass \sim 10 ^ { 8.5 } M _ { \odot } .
Gas-phase metallicities measured from available spectra are also low .
ELGs in our sample present bluer colours in the UVJ plane than the median colour-selected star-forming galaxy in SHARDS .
We suggest a new ( V-J ) colour criterion to separate ELGs from non-ELGs in blue galaxy samples .
In addition , several galaxies present high densities of O-type stars , possibly producing galactic superwinds , which makes them interesting targets for follow-up spectroscopy .

Conclusions : We have demonstrated the efficiency of SHARDS in detecting low-mass ELGs ( \sim 2 magnitudes deeper than previous spectroscopic surveys in the same field ) .
The selected sample accounts for 20 % of the global galaxy population at this redshift and luminosity , and is characterized by young SF bursts with sub-solar metallicities and low extinction .
However , robust fits to the full SEDs can only be obtained including an old stellar population , suggesting the young component is built up by a recent burst of SF in an otherwise old galaxy .