Both the central defenders have signed for the 2015 ISL champions on free transfers.
Brazilian Mailson is joining after the end of his contract at Brazilian third tier outfit Volta Redonda while Portuguse Sereno has signed after a short spell with Spanish second tier outfit Almeria.
Mailson returns to the club having won the title in 2015 when he made 12 appearances. Sereno lifted the ISL trophy with Atletico de Kolkata last season and also scored in the final.
"Sereno has an incredible CV with immense experience and was one of the best defenders in last season's ISL. The duo adds strength in depth to our defence."
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"I can't express in words how I feel about returning home. I always dreamt of reuniting with the club and thank the owners and coach John Gregory for giving me this opportunity again. The fans remained incredibly supportive even though I played for another ISL team last year and now I want to do everything to bring back the trophy to Chennai for them," said Mailson.
29-year-old Mailson made nine appearances for NorthEast United in the 2016 ISL and then went on to join Volta Redonda in January 2017. He made 22 appearances for Volta this year before returning to Chennaiyin FC for a second spell.
In last season's ISL, Sereno made 10 appearances for ATK with his only goal being the equaliser in the final which the Kolkata side went on to win after a penalty shootout against Kerala Blasters.
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