The Ireland wing tweeted: "I hope my ears deceived me with some comments directed my way from the crowd. #Not on - Django wins in the end."

Django Unchained, a Quentin Tarantino film about an African-American slave, was released in 2012.

Asked for a response, Ulster Rugby said that they condemned all forms of abuse.

The Irish province's statement added that Ulster "will work with the relevant parties to robustly investigate any complaints received".

European Champions Cup organisers told Irish broadcaster RTE on Sunday afternoon that they had "not received a formal complaint from Racing 92 regarding any alleged case of verbal abuse of one of the club's players".

"EPCR [European Professional Club Rugby] is in contact with both Ulster Rugby and Racing 92, and will be making no further comment at this point," added the statement.

The wing, 28, scored a Racing try before being replaced at half-time and he was audibly booed whenever he had the ball.

Zebo's father is from the Caribbean island of Martinique while his mother is Irish.

The former Munster player's right leg was bandaged in the first half before he came off at Kingspan Stadium.

Zebo was involved in controversy when the teams played in Pool Four in Paris three months ago.

The Ireland international pointed his finger at Ulster full-back Michael Lowry before crossing to score Racing's fifth try in that game and was immediately told to apologise to his young opponent by referee Nigel Owens.

Following that game, Zebo issued a lengthy public apology to Lowry after giving his shirt to the Ulster player.

Ulster beat the French club 26-22 in Saturday's thrilling game in Belfast and both remain in contention to qualify for the Champions Cup knockout stages.