Disk of the Small Magellanic Cloud as traced by Cepheids
  The structure and evolution of the disk of the Small Magellanic Cloud (SMC)
are traced by studying the Cepheids. We aim to estimate the orientation
measurements of the disk, such as the inclination and the position angle of the
line of nodes, and the depth of the disk. We used the V and I band photometric
data of the fundamental and first-overtone Cepheids from the Optical
Gravitational Lensing Experiment survey. The period-luminosity relations were
used to estimate the relative distance and reddening of each Cepheid. A
weighted least-square plane fitting method was then applied to estimate the
structural parameters. The line-of-sight depth and then the orientation
corrected depth or thickness of the disk were estimated from the relative
distance measurements. The period-age-colour relation of Cepheids were used to
derive the age of the Cepheids. A break in the PL relations of the
fundamental-mode and first-overtone Cepheids at P ~ 2.95 days and P ~ 1 day are
observed. An inclination of 64$^o$.4$\pm$0$^o$.7 and a
PA$_{lon}$=155$^o$.3$\pm$6$^o$.3 are obtained from the full sample. A reddening
map of the SMC disk is also presented. The orientation-corrected depth or
thickness of the SMC disk is found to be 1.76 $\pm$ 0.6 kpc. The scale height
is estimated to be 0.82 $\pm$ 0.3 kpc. The age distribution of Cepheids matches
the SMC cluster age distribution. The radial variation of the disk parameters
mildly indicate structures/disturbances in the inner SMC (0.5 $<$ r $<$ 2.5
degree). Some of the Cepheids found in front of the fitted plane in the eastern
regions are possibly the youngest tidally stripped counterpart of the H {{\sc
i}} gas of the Magellanic Bridge. The Cepheids behind the fitted plane are most
likely the population in the Counter Bridge predicted in recent numerical
simulations. Different scenarios for the origin of the extra-planar Cepheids
are also discussed.
