8. The Sun Letters.
Due to a Phonological rule, the /ل / sound of the definite article is assimilated by any of the following sun consonants. Therefore, you need to use Shadda to replace the assimilated /ل /. The Sun Letters are:
ت ث د ذ ر ز س ش ص ض ط ظ ن الدَّرسُ