Austria signed as a member state of CERN on 1 June 1959. The press release announcing the accession noted:
Following applications made in 1958 by the Austrian government, the council agreed unanimously to accept Austria as the 13th European member state to participate in CERN. Welcoming Mr W Goertz, permanent representative of Austria to the UN, MF de Rose, president of CERN council said:
A country where cosmic rays were discovered and which gave such names as Hess, Boltzmann, Schrödinger and Pauli to physics has its natural place in CERN. The difficult post-war period only, M de Rose pointed out, prevented Austria from joining earlier. We are happy that the accession of Austria now marks the end of this period of post-war difficulties and the beginning of a new contribution of that country to international cooperation and European culture.
The release notes that the Austrian permanent representative was "particularly pleased" to see Austria's flag together with those of the other member states already flying when he arrived at the CERN entrance for the afternoon session of the council.
Read the press release here.