يَآ أَيُّهَا الكَافِرَوْنَ

يَآ أَيُّهَا الكَافِرُوْنَ is a phrase (شِبْهُ الجُمْلَةِ) consisting of a particle of calling, a particle of attention and a definite noun

PhraseParticle/Particle/NounNounParticleParticle
شِبْهُ الجُمْلَةِ حَرْفُ النِّدَآءِ وَحَرْفُ التَّنْبِيهِاسْمٌ مَعْرِفَةٌحَرْفُ التَّنْبِيهِحَرْفُ النِّدَآءِ
PhraseParticle, particle and definite noundefinite nounparticle of attentionparticle of calling 
  
O disbelieversيَآ أَيُّهَا الكَافِرُوْنَ=الكَافِرُوْنَ+أَيُّهَا+يَا
 O disbelieversdisbelievers O
Surah/Below is the only verse in the holy Quran containing the compoundيَآأَيُّهَا الكَافِرُوْنَ
Verse
109-1Say, O Disbelieversقُلْ يَآ أَيُّهَا الكَافِرُوْنَ