  We investigate the effect of Fe II equivalent width ($W_{2600}$) and fibre
size on the average luminosity of [O II]$\lambda\lambda$3727,3729 nebular
emission associated with Mg II absorbers (at $0.55 \le z \le 1.3$) in the
composite spectra of quasars obtained with 3 and 2 arcsec fibres in the Sloan
Digital Sky Survey. We confirm the presence of strong correlations between [O
II] luminosity (L$_{[\rm O~II]}$) and equivalent width ($W_{2796}$) and
redshift of Mg II absorbers. However, we show L$_{[\rm O~II]}$ and average
luminosity surface density suffers from fibre size effects. More importantly,
for a given fibre size the average L$_{[\rm O~II]}$ strongly depends on the
equivalent width of Fe II absorption lines and found to be higher for Mg II
absorbers with $R \equiv$ $W_{\rm 2600}/W_{\rm 2796}$ $\ge 0.5$. In fact, we
show the observed strong correlations of L$_{[\rm O~II]}$ with $W_{2796}$ and
$z$ of Mg II absorbers are mainly driven by such systems. Direct [O II]
detections also confirm the link between L$_{[\rm O~II]}$ and $R$. Therefore,
one has to pay attention to the fibre losses and dependence of redshift
evolution of Mg II absorbers on $W_{2600}$ before using them as a luminosity
unbiased probe of global star formation rate density. We show that the [O II]
nebular emission detected in the stacked spectrum is not dominated by few
direct detections (i.e., detections $\ge 3 \sigma$ significant level). On an
average the systems with $R$ $\ge 0.5$ and $W_{2796}$ $\ge 2$ \AA\ are more
reddened, showing colour excess E($B-V$) $\sim$ 0.02, with respect to the
systems with $R$ $< 0.5$ and most likely traces the high H I column density
systems.
