In this paper, the author compares the monophthongization of ai/ay and au/aw in Old Germanic dialects and Arabic dialects. The main question is whether the monophthongizations of ai and au into ē and ō in the Germanic and Arabic dialects are phonologically comparable. This turns out to be the case to a large extent. In both linguistic groups there were biphonematic diphthongs in the protophase, followed by a monophonematic period, after which monophthongization could occur. Moreover, in both groups there were dialects without monopthongization and other dialects in which monophthongization was not finished and intermediate forms like ei and ou occurred. Another question is whether contacts between Germans and Arabs played a part in mutual monophthongization. This turns out to be highly unlikely.
Intriguingly, a different monophthongization should be taken into account as well: au and ai into ā and a, which also occurs in both dialect groups. I have conformed to the Arabist terminology for distinguishing between these monophthongizations: monopthongizations into ē and ō are called contraction monophtongizations, those into ā and a they call elision monophthongizations.
Blanc1981: H.Blanc, “Egyptian Arabic in the Seventeenth Century. Notes on the Judeo- Arabic Passages of Darxe Noʾam (Venice, 1697),” in: Studies in Judaism and Islam. Presented to Shelomo Dov Goitein on the Occasion of his Eightieth Birthday by his Students, Colleagues, and Friends, Jerusalem: 185–202.
Lévi-Provençalet al.2012: E.Lévi-Provençal, J. D.Latham, L.Torres Balbás and G. S.Colin, “al-Andalus,” in: Encyclopaedia of Islam, 2nd ed., eds. P.Bearman, Th.Bianquis, C. E.Bosworth, E.van Donzel and W. P.Heinrichs, http://dx.doi.org/10.1163/1573–3912_islam_COM_0054.
Melvinger2012: A.Melvinger, “al-Madjūs,” in: Encyclopaedia of Islam, 2nd ed., eds. P.Bearman, Th.Bianquis, C. E.Bosworth, E.van Donzel and W. P.Heinrichs, http://dx.doi.org/10.1163/1573-3912_islam_SIM_4752.