  We present a detailed photometric and spectroscopic analysis of DD CMa, based
on published survey photometry and new spectroscopic data. We find an improved
orbital period of $P_\mathrm{o}= 2.0084530 \pm 0.0000006 ~\mathrm{d}$. Our
spectra reveal H$\beta$ and H$\alpha$ absorptions with weak emission shoulders
and we also find color excess in the WISE multiband photometry, interpreted as
signatures of circumstellar matter. We model the $V$-band orbital light curve
derived from the ASAS and ASAS-SN surveys, assuming a semidetached
configuration and using the mass ratio and temperature of the hotter star
derived from our spectroscopic analysis. Our model indicates that the system
consists of a B 2.5 dwarf and a B 9 giant of radii 3.2 and 3.7
$\mathrm{R_{\odot}}$, respectively, orbiting in a circular orbit of radius 6.75
$\mathrm{R_{\odot}}$. We also found $M_{\mathrm{c}} = 1.7 \pm 0.1
~\mathrm{M_{\odot}}$, $T_{\mathrm{c}} = 11350 \pm 100 ~\mathrm{K}$ and
$M_{\mathrm{h}} = 6.4 \pm 0.1 ~\mathrm{M_{\odot}}$, $T_{\mathrm{h}} = 20000 \pm
500 ~\mathrm{K}$, for the cooler and hotter star, respectively. We find broad
single emission peaks in H$\alpha$ and H$\beta$ after subtracting the synthetic
stellar spectra. Our results are consistent with mass exchange between the
stars, and suggest the existence of a stream of gas being accreted onto the
early B-type star.
