We present B and V time series photometry of the M31 satellite galaxy Andromeda XXVII ( And XXVII ) that we observed with the Large Binocular Cameras of the Large Binocular Telescope .
In the field of And XXVII we have discovered a total of 90 variables : 89 RR Lyrae stars and 1 Anomalous Cepheid .
The average period of the fundamental mode RR Lyrae stars ( RRab ) \langle P _ { ab } \rangle =0.59 d ( \sigma =0.05 d ) and the period-amplitude diagram place And XXVII in the class of Oosterhoff I/Intermediate objects .
Combining information from the color-magnitude diagram ( CMD ) and the variable stars we find evidence for a single old and metal poor stellar population with [ Fe/H ] \sim - 1.8 dex and t \sim 13 Gyr in And XXVII .
The spatial distribution of RR Lyrae and red giant branch ( RGB ) stars gives clear indication that And XXVII is a completely disrupted system .
This is also supported by the spread observed along the line of sight in the distance to the RR Lyrae stars .
The highest concentration of RGB and RR Lyrae stars is found in a circular area of 4 arcmin in radius , centered about 0.2 degrees in south-east direction from Richardson et al .
( 44 ) center coordinates of And XXVII .
The CMD of this region is well defined with a prominent RGB and 15 RR Lyrae stars ( out of the 18 found in the region ) tracing a very tight horizontal branch at \langle V ( RR ) \rangle = 25.24 mag \sigma = 0.06 mag ( average over 15 stars ) .
We show that And XXVII well proposes as a candidate building block of the M31 halo .