  We follow up emission line galaxies identified through the near-infrared
slitless HST/WFC3 WISP survey with VLT/FORS2 optical spectroscopy. Over 4 WISP
fields, we targetted 85 of 138 line emission objects at $0.4<z<2$ identified in
WFC3 spectroscopy. Half the galaxies are fainter than $H_{AB}=24$mag, and would
not have been included in many well-known surveys based on broad-band magnitude
selection. We confirm 95% of the initial WFC3 grism redshifts in the 38 cases
where we detect lines in FORS2 spectroscopy. However, for targets which
exhibited a single emission line in WFC3, up to 65% at $z<1.28$ did not have
expected emission lines detected in FORS2 and hence may be spurious (although
this false-detection rate improves to 33% using the latest public WISP emission
line catalogue). From the Balmer decrement the extinction of the WISP galaxies
is consistent with $A($H$\alpha)=1$mag. From SED fits to multi-band photometry
including Spitzer $3.6\mu$m, we find a median stellar mass of
$\log_{10}(M/M_{\odot})=8.94$. Our emission-line-selected galaxies tend to lie
above the star-forming main sequence (i.e. higher specific star formation
rates). Using [OIII], [OII] and H$\beta$ lines to derive gas-phase
metallicities, we find typically sub-solar metallicities, decreasing with
redshift. Our WISP galaxies lie below the $z=0$ mass-metallicity relation, and
galaxies with higher star formation rates tend to have lower metallicity.
Finally, we find a strong evolution with redshift of the H$\alpha$ rest-frame
equivalent width, $EW_0($H$\alpha)\propto (1+z)^{1.85\pm0.24}$ with higher
$EW_0$ galaxies having larger [OIII]/H$\beta$ and O32 ratios on average,
suggesting lower metallicity or higher ionisation parameter in these extreme
emission line galaxies.
