  Numerous EWs were discovered by several deep photometric survey and there are
about 40785 EW-type binary systems listed in the international variable star
index (VSX) by March 13, 2017. 7938 of them were observed by LAMOST by November
30, 2016 and their spectral types were given. Stellar atmospheric parameters of
5363 EW-type binary stars were determined based on good spectroscopic
observations. In the paper, those EWs were catalogued and their properties are
analyzed. The distributions of the orbital period (P), the effect temperature
(T), the gravitational acceleration (Log(g)), the metallicity ([Fe/H]) and the
radial velocity (RV) are presented for those observed EW-type systems. It is
shown that about 80.6\% sample stars have metallicity below zero indicating
that EW-type systems are old stellar population. This is in agreement with the
the conclusion that the EW binaries are formed from moderately close binaries
through angular momentum loss via magnetic braking that takes a few hundred
million to a few billion years. The unusual high metallicities of a few percent
of EWs may be caused by contaminating of material from the evolution of unseen
neutron stars and black holes in the systems. The correlations between the
orbital period and the effect temperature, the gravitational acceleration and
the metallicity are presented and their scatters are mainly caused by (i) the
presence of the third bodies and (ii) the wrong determined periods sometimes.
It is shown that some EW contain evolved component stars and the physical
properties of EWs are mainly depending on their orbital periods. It is found
that the extremely short-period EWs may be older than their long-period cousins
because they have lower metallicities. This reveals that they have a longer
timescale of pre-contact evolution and their formation and evolution are mainly
driven by angular momentum loss via magnetic braking.
