Interrogative pronouns are compound expressions; the question element hu or o may be absent with 'who', 'what', and, exceptionally, 'how' however.
Source: Schumann 1783: 69
Source: Schumann 1783: 68
Source: van Dyk ca. 1765: 34
Source: Schumann 1783: 149
Source: van Dyk ca. 1765: 23
Source: Bruyn 1995a: 145
Source: Herlein 1718: 122
Source: Herlein 1718: 122
Source: van Dyk ca. 1765: 70
Source: Weygandt 1798: 91