For the past few months, Margarete Kraus’s face has been looking out at passengers in the lifts at Russell Square tube station. Photographed in the 1960s, she is leaning from the window of her caravan, smiling. Her Auschwitz prisoner number is tattooed on her left forearm.

Kraus, who came from Czechoslovakia, was one of the hundreds of thousands of Roma and Sinti people targeted by the Nazis for extermination in the 1940s. Her story is told, alongside those of others, by an exhibition at the Wiener Holocaust Library (until 11 March). The exhibition carefully explains that meanings of the term Holocaust vary: ‘The genocide against the Roma can be defined as a part of the Holocaust. Sometimes, however, the term Holocaust is used only to mean the systematic murder of six million European Jews between 1941 and 1945.’ A Romani word for the genocide is porajmos, or ‘the devouring’.

What isn’t in dispute is that between 220,000 and 500,000 Roma and Sinti people were murdered by the Nazis, on explicitly racist grounds. Many, including survivors such as Kraus, were subjected to forced medical experiments. As the exhibition makes clear, part of the problem with recognising what Germany did is the fact that so many of the victims were already marginalised. There were few accurate records of Roma and Sinti populations before the Second World War, and many of the massacres by the Nazis and their collaborators went unrecorded. Prejudice and discrimination against the Roma has existed in Europe for centuries, and governments were using modern techniques to control and subjugate them before Hitler gained power. The Weimar Republic launched measures to target ‘Gypsies, vagabonds and the work-shy’, passing a law in 1929 that required all Roma and Sinti to register with the police.

Discrimination of this sort is about more than simple bigotry; it is intimately linked with the way in which states try to count and to categorise people. When many of Europe’s modern nation-states were being formed, the Roma were excluded from citizenship, either legally or in practice. The EU has been a mixed blessing: in some respects it has allowed Roma people to escape persecution by moving to countries where they are safer; in others it has made discrimination worse, since governments tend to treat them as interlopers, even if their passports give them the right to be there.

Hostility to Roma, Sinti and other Traveller groups remains prevalent. France has been pushing thousands of Roma back to Bulgaria and Romania for the last decade. Two years ago, fascist groups carried out a series of violent attacks on Roma living in Ukraine. And buried in the UK Conservative Party’s election manifesto last month was a promise to give the police new powers to arrest and seize the property of ‘trespassers who set up unauthorised encampments’.

Official recognition of the Roma genocide has been relatively recent. West Germany issued an apology in 1982; Angela Merkel unveiled the Memorial to the Sinti and Roma Victims of National Socialism in Berlin in 2012. France apologised for its role in helping the Nazis round up and deport Roma and Sinti in 2016.

But people have other ways of making themselves visible. Theresia Reinhardt, a former dancer at the Würzburg State Opera whose children were taken away from her and experimented on, ran a Sinti organisation in West Germany that campaigned for recognition of the genocide after the war. Researchers collected evidence of forced sterilisations and used it to prosecute Nazi war criminals. Survivors of the concentration camps gave testimony to historical researchers, some of which is displayed in the Wiener Library.

In one corner of the room, visitors are invited to leave their comments. ‘I am really glad of this exhibition,’ reads one. ‘As a Jew I feel it is incredibly important [that] I, other Jews and non-Jews learn more of the ‘”forgotten Holocaust”.’ Another, from a 10-year-old visitor, says it’s helped with his homework. Nearby is a picture of the Roma flag, adopted by the World Romani Congress in 1971: blue symbolises the sky, green symbolises the land, and a red wheel symbolises movement and progress. ‘It seems to me that the Roma flag is one that all nations could do well to adopt,’ a visitor has written.