Esperanto has about twenty dictionary entries ending in ‑AŬ, which form a sort of slapdash word class:
- adiaŭ = “goodbye” (formulaic interjection)
- almenaŭ = “at least” (adverb/modifier)
- ambaŭ = “both” (determiner/modifier)
- ankaŭ = “also/moreover” (adverb/modifier)
- ankoraŭ = “still” (adverb)
- anstataŭ = “instead of” (preposition)
- antaŭ = “before/in front of” (preposition)
- apenaŭ = “barely/scarcely” (adverb/conjunction/modifier)
- aŭ = “or” (coordinating conjunction)
- baldaŭ = “soon” (adverb)
- ĉirkaŭ = “around” (adverb/preposition)
- hieraŭ = “yesterday” (adverb/pronoun)
- hodiaŭ = “today” (adverb/pronoun)
- kontraŭ = “against/in return for/opposite” (preposition)
- kvazaŭ = “as if” (adverb/conjunction)
- laŭ = “according to/along” (preposition)
- malgraŭ = “despite/notwithstanding” (preposition)
- morgaŭ = “tomorrow” (adverb/pronoun)
- naŭ = “nine” (regular number)
- preskaŭ = “almost/nearly” (adverb/modifier)
(Yes, as far as I'm concerned almost in almost ninety is a degree modifier; both in both feet is a quantifying determiner; and tomorrow in until tomorrow is a temporal deictic pronoun. If you disagree, that's fine; go ahead and believe that all words are adverbs, or whatever your theory is – you don't need to write and tell me about it.)
A few of these words, most obviously aŭ, are only on the list due to ending in the same letters by coincidence – whatever that means in a designed language. On the other hand the list can also be extended by coinages like antaŭhieraŭ = “the day before yesterday”, kajaŭ = “and/or”, and malantaŭ = “behind”, not to mention the jokey back‐formation graŭ = “due to”.
Unlike the family of “regular” pronouns that all end in ‐I but nonetheless count as bare roots, Zamenhof didn't cover this one in the Fundamento's sixteen rules. However, he did confirm elsewhere that he thought of the ‑AŬ as an added ending: the roots are really adi‑, almen‑, amb‑, and so forth (though the temporal/spatial preposition ant‑ collides awkwardly with the present progressive participle morpheme ‑ant). We even have his permission to replace the ending with an apostrophe, if we think malgr’ sounds more euphonious!
The problem with this word‐class is that its members have no unifying property other than having been dumped in the same wastebasket; some of them could have been created as regular adverbs ending in ‐E, while most of the others might as well have been bare roots like the rest of the language's prepositions, conjunctions, and miscellaneous odds and ends. The whole thing is just an inadequately considered first draft of something Zamenhof never quite got round to either turning into a consistent feature of the language or tidying away – compare the vestigial ‑es genitive case, which he mostly discarded but which left traces on the “correlatives”. Considering almost half of the words on the list above begin with A‐, it's easy to suspect that he gradually lost confidence in the idea as he was compiling his way through his dictionary.
Tacking word‐class markers onto any of these words can produce a regular adjective, verb, or whatever, capable of taking further affixes of its own. Such derivatives have one feature otherwise rare in Esperanto: a syllable‐initial Ŭ, as in anta ŬU lo, kontra ŬA ĵoj, la ŬON ta = “a predecessor, setbacks, about to conform”. Or are we supposed to be dividing the syllables after the Ŭ to give antaŭ ʾU lo, kontraŭ ʾA ĵoj, laŭ ʾON ta? That would be odd behaviour if it was true that AŬ is just a normal vowel‐plus‐consonant sequence – but then again if that was the case, why would Zamenhof have treated the ‑AŬ ending as parallel to all the other word‐class markers that are single vowel sounds?